1. Filed of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having the easy magnetization axis in the perpendicular direction to the surface of the medium.
2. Description of the Invention
A perpendicular type magnetic recording system in which the magnetic recording medium is magnetized in a direction perpendicular to the surface of the recording medium, is disclosed in Japanese Unexamined Patent Publication 134706/1977. Extensive researches are being made by various circles on the perpendicular type magnetic recording as a prospective system expected to be commercially available in near future as a super high density recording system.
It has generally been accepted that a Co-Cr layer formed by sputtering is most suitable as a recording medium for such a perpendicular type magnetic recording system.
During the course of the study of the Co-Cr layer from the standpoint of mass proudction and practical applications, however, the present inventors have found that there is a certain practical problem involved. Namely, when the deposition speed (i.e. the sputtering speed) is increased in an attempt to reduce the proudction costs, the coercive force (Hc (.perp.)) tends to increase correspondingly, thus leading to a difficulty that no adequate overwrite characteristic is thereby obtainable in the actual digital recording.
The overwrite characteristic must be at least 25 dB. To obtain the overwrite characteristic of such a level, the coercive force (Hc (.perp.)) must be at most 600 Oe.
When the deposition speed (i.e. the sputtering speed) is at a low level (i.e less than several hundreds .ANG./min), it is possible to produce a Co-Cr layer having a coercive force (Hc (.perp.)) of less than 600 Oe by controlling the cooling of the substrate. However, the production costs by such an inefficient operation become extremely high and it is impossible to practice the production on an industrial scale.
At an industrially feasible deposition speed (i.e. greater than several thousands .ANG./min), the coercive force (Hc (.perp.)) inevitably becomes to be at least 600 Oe even when the cooling efficiency of the substrate is raised to the maximum level, and it sometimes reaches a level as high as 2000 Oe in an extreme case.